In order to improve the quality of Dense Wavelength Division multiplexing (DWDM) transmission dedicated for long haul distances (more than 1000 kilometers), different solutions have been proposed based on the use of modulation format such as Non Return-to-Zero (NRZ), Return-to-Zero (RZ), Carrier-Suppressed Return-to-Zero (CS-RZ), Return-to-Zero Differential Phase Shift Keyed Signals (RZ-DPSK), Phase Shaped Binary Transmission (PSBT) etc. . . . All these techniques try to reduce the impact of propagation effect occurring usually in DWDM environment among other by reducing the spectral width of each channel.
In conventional manner, the power spectrum density of RZ optical signal is relatively broad because of the large number of transitions in the signal to be transmitted. Due to the spread over a wide range of frequencies of the transmitted energy, an RZ signal is sensitive to group velocity dispersion i.e. to chromatic dispersion, and also to four-wave mixing (FWM) or “cross-talk” in DWDM systems. Nevertheless, RZ format presents the advantage of being little affected by self-phase modulation (SPM) in comparison to a NRZ format. It often happens that the SPM induced by optical non-linearities in a line fiber gives rise to optical signal distortion that reduces the range and the capacity of optical transmission systems. In addition, RZ signals are suitable for being regenerated by synchronous modulation.
Conversely, the power spectrum density of a NRZ optical signal is narrower than that of a RZ signal. However, in NRZ format, both capacity and transmission range are limited by SPM. Furthermore, no optical or electronic regenerators exist that are capable of processing such signals at high bit rates. Such signals are not easily integrable and introduce losses because of the interaction between successive “0” and “1” bits, and/or distortion, so that the extinction ratio of the signal after electrical filtering is degraded. There exist also CS-RZ optical signals having the property of presenting bits that are always phase-shifted by 180° relative to adjacent bits. Such CS-RZ signals possess numerous advantages over conventional signals like RZ and NRZ. More particularly, the interaction between adjacent bits are reduced due to the different phase between neighbor bits. Therefore, the use of CS-RZ reduces intrachannel effects, one of the main limitation for optical transmission rates at or above 40 Gbit/s.
In an article entitled “40 Gbit/s L-band transmission experiment using SPM-tolerant carrier-suppressed RZ format”, published in Elec. Letters, Vol. 35, No. 25, Dec. 9, 1999, p. 2213 A. Hirano et al. describe using a shifted dispersion optical fiber link in particular, a study of the optimum dispersion stabilities between RZ, CS-RZ, and NRZ signals in the large (L) transmission band at frequencies in the range between 1570 nanometers (nm) to 1605 nm. It appears that CS-RZ signals at 40 Gbit/s present the most stable optimum dispersion and remain the closest to a total dispersion in the vicinity of 0 picosecond per nanometer (ps/nm). Dispersion tolerance is explained in particular by the phase inversion between adjacent bits which eliminates all inter-bit interference. Furthermore, CS-RZ signals subject the sensitivity of the receiver to little degradation at high power. Those results also confirm that CS-RZ signals are less sensitive to SPM than are NRZ signals. In this article, the generator producing the CS-RZ optical signals at 40 Gbit/s comprise a Mach-Zehnder modulator in push-pull mode fed with a sinusoidal electrical signal of 20 gigahertz (GHz). The CS-RZ pulse width takes 66% of the time bit (16.5 ps at 40 Gbit/s). It is usually obtained using half frequency driving (typically 20 GHz) of the modulator biased at the minimum of the transfer curve.
Another type of CS-RZ clock signal generator (transmitter) is based on using a phase modulator to change the phase of each successive bit. Due to their limited passbands, those prior art generators or transmitter do not make it possible, at present, to produce stable CS-RZ signals at a modulation frequency exceeding 40 Gbit/s. In other words, such generators or transmitters are unsuitable for producing CS-RZ signals at very high bit rates.